A vigil will be held in Castlerea this evening to remember Detective Garda Colm Horkan.
His colleagues and members of the local community will gather at Fairgreen at 7.30pm to remember the 49-year-old.
Earlier, Taoiseach Leo Varadkar has said the “nation is grieving” over his killing in Roscommon last night.
Speaking to the media at Government Buildings, Mr Varadkar delivered a short address in which he described the 89th killing of an active member of An Garda Siochana as “tragic”.
Garda Horkan was killed late last night in Co Roscommon and a man in his 40s has been arrested and is being held at Castlerea Garda Station.
It is understood that he was on patrol when he stopped to speak to a man on a motorbike on Main Street in the town who had recently been in contact with Gardaí.
The conversation developed into a physical altercation, during which the man took possession of Garda Horkan's gun.
It is being reported the detective has shot at 15 times and pronounced dead at the scene.
The Taoiseach said: “Last night we lost a member of An Garda Siochana in the most tragic of circumstances and today a nation is grieving.
“I’ve spoken to the Garda Commissioner today and the Minister for Justice and we are in touch with the family of Detective Garda Colm Horkan to offer our condolences and support.
“I extend it also to all of his colleagues in Castlerea Garda Station, to every member of An Garda Siochana, his friends and all who knew him.
Speaking in Irish, Mr Varadkar said everyday members of An Garda Siochana put their lives on the line to keep the country safe, saying: "Every day our Gardaí put themselves on the frontline of crime prevention, on behalf of all of us. This requires regular acts of bravery and courage. Sometimes the outcome is tragic and a Garda makes the ultimate sacrifice in the course of their duties."
“Colm gave his life protecting the community he served with distinction and courage during his 24 years as a member of An Garda Siochana.
"Our Gardaí are our guardians of the peace and defenders of the rule of law and they are role models in our society.
“This tragedy is a heart-wrenching reminder of how much we owe our Gardai for the work they do all over the country. They protect and serve our communities, our citizens and our country every day putting themselves in the front line on behalf of all of us,” he said.
Finishing in Irish, Mr Varadkar said: "Tá ár smaointe leo siúd go léir atá faoi bhrón inniu de bharr na heachtra tragóidí seo."
Earlier, the parish priest in Castlerea, Fr John McManus, said there is deep shock and anguish in the town at the death of Detective Garda Colm Horkan.
Fr McManus said he was called to administer the last rites and that other garda members joined him in prayer at the scene of the shooting.
He said: “I was called by the guards last night and it was only when I got up there that I realised the seriousness of it and paramedics were trying to save his life. Sadly that was not to be so I was invited to give the last rites and say some prayers.
Tragic news from Castlerea this morning with the death of Garda in the line of duty. My heartfelt sympathy to his family, friends & all Gardai. Murder investigation under way. pic.twitter.com/tgAosMJZBa— Charlie Flanagan (@CharlieFlanagan) June 18, 2020
“I thought it was very poignant to say some prayers that people would know and all of Colm’s garda colleagues joined in.
“It was a horrendous incident and it had a huge effect on the community. Castlerea would be a close-knit town so understandably there is deep shock and anguish among people.”